The petrology and geochemistry of the Nordeste volcanic complex, São Miguel, Azores

• Main Author: Fernandez, Louis A.
• Publication Date: 1982
• Format: Article
• Language: eng
• Source: Repositórios Científicos de Acesso Aberto de Portugal (RCAAP)
• Download full: http://hdl.handle.net/10400.3/4906
• topic: Geochemistry; Petrology; São Miguel Island (Azores); Nordeste

Sixty samples from the Nordeste Volcanic Complex, São Miguel, Azores, have been analyzed for major elements and fifteen for LIL and other trace metals. Analysis of magnesia and alkali-silica variation diagrams show the bulk of the Nordeste volcanics to be a potassic, mildly alkaline, alkali basalt → traohybasalrt → tristanite → trachyte series. The oldest flows of the complex, characterized by low TiO² and high SiO², are interpreted as representative of compositions transitional between tholeiitic and alkalic suites. All of the Nordeste samples are enriched in lighter REE relative to Mid-Atlantic Ridge Basalts from the Azores Plateau. Light REE enrichment factors range through a factor of ~ 5 between the basaltic types and the tristanites and trachytes. The younger Nordeste volcanics, when compared to the transitional basalts, are distinguished by higher concentrations of Ce, Rb, Ba, Eu, Lu, Hf, and Yb and higher La/Sm ratios. The transitional basalts are characterized by high T/Th and Ta/Hf ratios. Using major, minor and trace element compositions and microprobe analyses of phenocrystic and groundmass phases, least-squares calculations support a crystal fractionation model. The magnesian aphyric to ankaramitic alkali basalts represent magmas from which minor amounts of olivine and pyroxene were either removed from or incorporated into while residing in shallow crustal reservoirs. The trachybasalt → trachyte series represent fractionated liquids generated by the removal of titanaugite, plagioclase, olivine, titanomagnetite, and ilmenite in the early stages follwed by removal of abundant plagioclase, augite and minor opagues in the late stages. The dominance of plagioclase in the latter fractionation scheme and its rarity as an xenocrystic phase suggests fractionation in deeper reservoirs, now represented by gabbroic and dioritic layered complexes underlying the Azores Platform. The major and minor element composition of the transitional basalts precludes their being the parent of the alkali basalt sequence, but rather indicate two magma series geenrated by diferrent degrees of partial melting in the mantle.